Method for coating a continuous web

ABSTRACT

Two or more low-coverage coatings are applied substantially simultaneously to a web travelling at high speed by first trowelling a very thin subcoat onto the web with a trailing blade coater to prepare the web surface for an overcoating of the same or compatible coating material, and then flowing the overcoat down the top side of the trailing blade and off the trowelling end thereof directly onto the subcoat. Two or more layers of fluid coating composition can be flowed down the top of the trailing blade and onto the trowelled subcoat in distinct layer relationship with each other and the subcoat.

United States atent Inventor Joseph A. Mercier Rochester, N.Y.

Appl. No. 55,323

Filed July 16,1970

Patented Dec. 14, 1971 Assignee Eastman Kodak Company Rochester, N.Y.

METHOD FOR COATING A CONTINUOUS WEB 2 Claims, 8 Drawing Figs.

US. Cl 117/69,

117/34,117/761 ,117/83,1l7/l55R,1l7/156, 117/161 UE, 117/164, 118/126,118/412 [56] References Cited UNITED STATES PATENTS 2,932,855 4/1960Bartlett et al. 118/412 X 3,442,684 5/1969 Woodham et al 1 18/126 XPrimary Examiner-Alfred Lv Leavitt Assistant Examiner-Edward G. WhitbyAttorneys-Walter O. Hodsdon and Karl T. Naramore ABSTRACT: Two or morelow-coverage coatings are applied substantially simultaneously to a webtravelling at high speed by first trowelling a very thin subcoat ontothe web with a trailing blade coater to prepare the web surface for anovercoating of the same or compatible coating material, and then flowingthe overcoat down the top side of the trailing blade and off thetrowelling end thereof directly onto the subcoat. Two or more layers offluid coating composition can be flowed down the top of the trailingblade and onto the trowelled subcoat in distinct layer relationship witheach other and the subcoat.

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@RAD/US INVENTOR RAD/US T05 BLADE TH/C/(NESS ATTORNEYS METEOD FORCOATING A CONTINUOUS WEB The present invention relates to coating acontinuous web, and particularly to a method and apparatus for applyingvery low coverage with reasonable uniformity in thickness to a webtravelling at high speed.

A very difficult coating problem is the application of very lowcoverages with reasonable thickness uniformity to a continuous webtravelling at high speed. One approach which is used in the commercialcoating field is the trailing blade coater. This device, however,operates as a smoothing coater, filling the low areas of the web surfaceand scraping clean the high points. The resulting coating is notsatisfactory for use in arts, e.g., the photographic am, where areasonable uniformity of thickness of coating is desired, although it isvaluable as a surface coater for the stock. One problem encountered withtrailing blade coaters is that the fluid coating composition has atendency to dry on the blade edge and eventually a. buildup of solidcoating material is formed on the blade edge which produces longitudinalstreaks in the coating.

Slide coaters have been used in the art of coating photographic films orplates and wherein the emulsion flows down an inclined surface whichextends from a hopper to the surface to be coated, see U.S. Pat. No.401,771. U.S. Pat. No. 2,761,417 shows a multiple slide hopper by meansof which two or more layers of fluid coating composition can besimultaneously applied to a continuous web in distinct layerrelationship. While these slide coating techniques are capable ofapplying reasonably uniform coatings to a continuously moving web, theyare somewhat limited in coating speed because at high speed air tends tobecome entrained beneath the coating as it comes off the slide or iscoated from a bead, as the case may be.

The primary object of the present invention is to provide a method ofcoating which combines the trailing blade and slide methods of coatingin such a way as to overcome the shortcomings of both and provide amethod by which one or more coating compositions of very low coveragesand of reasonable thickness uniformity can be applied to a continuousweb at high speeds.

Another object is to provide a method of applying very low coveragecoatings to a web travelling at high speed which consists in firsttroweling a very thin subcoat onto the web surface with a trailing bladecoater to prepare the surface for an overcoating of the same orcompatible coating material, and then flowing the overcoat down the topside of the trailing blade and off the troweling end thereof directlyonto the subcoat.

A further object is to provide a method of coating as described above bythe use of which two or more layers of fluid coating composition(s) canbe flowed down the top surface of the trailing blade and be floweddirectly onto the trowelled subcoat in distinct layer relationship witheach other and the subcoat.

And yet another object is to provide a novel coating apparatus forcarrying out the above-described methods of coating, and which l havechosen to designate as a trailing blade slide coater.

The novel features that 1 consider characteristic of my invention areset forth with particularity in the appending claims. The inventionitself, however, both as to its organization and its methods ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings in which:

FlG. 1 is a schematic view of a conventional trailing blade coater, andshowing the system parameters thereof;

FIG. 2 is a side view, partly in section, showing a two-layer trailingblade slide coater constructed in accordance with one embodiment ofthepresent invention;

FlGS. 3 and 4 are views similar to FIG. 2, but showing other ways inwhich the undercoat or subcoat may be applied to the web;

FIG. 5 is a view similar to FlGS. 2-4, but showing another embodiment ofthe apparatus adapted to apply three layers of coating to a web indistinct layer relationship; and

FIGS. 6, 7 and 8 are schematic views showing different blade edgegeometries on an enlarged scale.

Generally speaking, the present method of applying a reasonablyuniformly thin layer of a given coating solution to the surface of a webmoving continuously at high speed comprises the steps of applying afluid composition to the surface of the web, troweling said compositionas a subcoating on the web surface by moving the web past the end of aflexible stationary blade pressing against the web surface to fill anylow areas in the web surface and providing a smooth wet surface on theweb; then continuously metering a given quantity of said fluidcomposition, or another coating composition compatible with saidsubcoat, in the form of a layer onto the uppermost surface of saidtrailing blade at a point spaced above the flexible end thereof, saidlayer of coating composition, under the influence of gravity, flowingdown said trailing blade to form a uniformly thin layer which ultimatelyflows off the end of the blade and onto the surface of the subcoat; andthen simultaneously curing the subcoat and the thin layer of fluidcoating composition. Two or more layers of fluid composition may besimultaneously applied to the subcoat in distinct layer relationshipwith each other and the subcoat if desired. The blade applied subcoatacts as a seal to prevent air entrainment under the subcoat(s) which isa major problem in slide or bead coaters when operating at high speeds.The moving liquid layer(s) flowing off the trailing edge of the bladealso acts to improve the blade operation by preventing drying of coatingmaterial on the blade edge.

Referring to FIG. 1, a conventional blade coater is sche matically shownto clarify the system parameters of such a coater as they apply to thepresent invention as will be described below. The web, W, to be coatedis supported in a smooth condition on the surface of a coating roll 10moving in a counterclockwise direction as shown by arrow 11. The web tobe coated may be raw paper, precoated paper, plastic film base, etc.Riding on the exposed surface of the web W as it moves around saidcoating roll is the flexible end 12 of a trailing blade 13 whose otherend 13" is clamped, or otherwise fastened, to a rigid support 14. Thetrailing blade may be made of any suitable springy material, e.g.,stainless steel, blue steel, etc., but it should be made of a materialwhich will not be readily corroded by the fluid composition to be usedand should be of such hardness that the trailing edge will not be undulyworn away by the rubbing contact it has with the web. v

The surface of the coating roll 10 may be metal or it may have aresilient surface which is engaged by the web.

In conventional operation of such a trailing blade coater, an excess ofa fluid coating composition is applied to the web surface ahead of theblade and a puddle of the composition builds up ahead of, nd adjacentto, the blade across its entire width. The flexible end of the bladethen trowels the composition which passes beneath the trailing edgethereof and acts as a smoothing coater, filling in the low areas of theweb and scraping clean the high points. The troweling action of thetrailing blade can be adjusted by varying the blade cantilever, thestiffness of the blade, and the coating angle of the blade, eachindividually or any two or three of these parameters in combination. Inother words, it will be appreciated that the more pressure that thetrailing edge of the blade exerts upon the surface of the web the lesscoating composition will flow beneath it. As will appear later, theamount of coating composition which will flow past the blade will alsodepend upon the physical characteristics of the coating composition. Itwill be appreciated that unless a perfectly flat web is moving past thetrailing blade the thickness of the coating trowelled onto the web willnot be uniformly thick. in other words, a trailing blade coater isprimarily useful only in filling the lower areas of a web surface andconditioning the web surface so that it is smooth.

Looking now at FlG. 2, a trailing blade slide coater for applying twolayers of coating solution to the surface of a web, and constructed inaccordance with one embodiment of the present invention, is shown andwill be described. The object here is to obtain a layer of a fluidcoating composition on the web which is thin and uniformly thick. To dothis, a subcoat is first applied to the web surface by a trailing bladecoater to provide a smooth wet surface on the web onto which the finalcoating of desired uniform thickness is applied and/or to condition thesurface of the web to receive and hold the final overcoat. This subcoatcan be the same composition as the overcoat, or it can be a differentcomposition, If it is a different coating composition, then the onlyrequirement is that it be compatible with the overcoat composition sothat the overcoat layer(s) will readily spread and adhere to the subcoatand not be adversely affected by the subcoat subsequent to theirapplication.

As in the previously described trailing blade coater, the web W is heldin wrapped relation with a coating roller ill to keep it smooth as itapproaches and passes the coating point, A trailing blade 13' is clampedbetween two sections l6 and i7 ofa coating hopper held together bysuitable means, e.g. bolts (not shown). The hopper section 17 isprovided with a cavity 18, one wall of which is defined by theclamped-end E3" of the trailing blade. The fluid subcoat composition,Sc, is fed into the cavity 18 through an inlet 19 by a pump P. Thesubcoat composition exits in a layer L through a discharge slot 20, theupper side of which is defined by the lower side of the trailing blade.As a result, the layer L of the subcoat composition flows down the underside of the trailing blade to form a puddle or bead 21 extendingcompletely across the deflected end 12 of the blade and between it andthe web. As the web moves across this puddle or bead 21, it tends tocarry some of the subcoat with it, and this is trowelled over the websurface by the deflected end of the blade. This trowelling action of theblade provides a smooth wet surface on the web since the blade causesthe subcoat to flll in the low areas of the web and scrapes clean thehigh points.

The hopper section 16 has a cavity 22 formed therein which is covered bya third hoppersection 23 fastened to hopper section 16 in any suitablemanner, e.g. by bolts (not shown). Leading from the cavity 22 there is adownwardly inclined, elongated discharge slot 24 formed between spacedfaces of hopper sections 16 and 23. The discharge slot 24 will be thesame length as the width of the web to be coated and its exit isdirectly obliquely onto the uppermost surface of the blade 13' at apoint spaced above its deflected end 12'. The fluid coating composition,00, which is to be coated onto the web in a thin layer of uniformthickness if fed into the cavity 22 through an inlet 22' by a pump P ofthe constant discharge type. As the overcoat composition c, is fed intothe cavity at a constant rate it will exit from the discharge slot 24 inthe form of a ribbon or layer onto the uppermost surface of thedownwardly inclined trailing blade down which it flows by gravity toform a layer L This layer L as it flows down the uppermost surface ofthe blade becomes uniformly thin before it slides off the end of theblade and onto the surface of the subcoat which has been trowelled onthe web surface by the blade. The deflected end 12' of the trailingblade intersects the web surface at an obtuse angle so that the layerl1. of overcoat flowing off the end thereof will flow onto the layer ofsubcoat in such a way as to maintain a distinct layer relationshipbetween the two. This distinct layer relationship between the twocoatings is not important if the two coating compositions are of thesame material. but it is important in certain cases where the coatingcompositions are different and a distinct layer relationship betweenthem is desired, eg in certain photographic color materials where theseveral color sensitive and filter layers must be applied in distinctlayer relationship.

The puddle 21 of the subcoat composition formed between the web surfaceand the under side of the trailing blade will prevent the entrainment ofair beneath the overcoat layer which slides off the end of the blade andonto the subcoat. At the same time, the trowelling action of the bladeon the subcoat composition will further prevent air entrainment underthe subcoat. It will thus be seen that this method of coating overcomesthe air entrainment problem which is most prevalent in high speedcoating procedures. Also, since there is a layer L, of overcoatcomposition continuously flowing off the trailing end of the blade, thetrailing blade operation is improved by the prevention of drying ofcoating material on the blade edge.

The method used in applying the subcoat solution was found to have amajor influence on the coating procedure. The method of application bestsuited for a particular situa tion depends mainly upon the properties ofthe subcoat composition to be applied, as will be more fully set forthbelow.

FIG. 3 shows a two-layer coating system similar to that shown in FIG. 2but having a different type of subcoat applicator, In this embodimentthe subcoat composition is applied to the web surface ahead of thetrailing blade 13' from a manifold applicator 30 extending transverselyof the web and having one, or more, inlets 3ll into which the fluidcoating composition is fed by a pump, not shown. The manifold applicatoris provided with a plurality of discharge openings 32 spaced along itslength through which the coating material issues in the form ofindividual streams spaced across the width of the web surface. Thesestreams of coating material deposited on the web build up into a puddle21 behind the trailing blade, said puddle serving to distribute thecoating material uniformly across the web immediately ahead of theblade.

MG. 4 shows another embodiment of a two-layer system similar to thatshown in FIGS. 2 and 3 but having a different type of subcoatapplicator. In this embodiment the subcoat is supplied from areservoir-type applicator 35 wherein a relatively large quantity ofcoating material is maintained on the web surface immediately behind thedeflected end 12' of the trailing blade 13'. The coating material is fedinto this reservoir through an inlet 36 by means of a pump not shown.

The present method of coating a web can be used to simultaneously applya plurality of different fluid coating compositions in distinct layerrelationship at high speeds. In FIG. 5 an embodiment is shown which issuitable for applying three different layers of coating composition ontoa web surface at the same time. Here the web W to be coated is passedaround a coating roll 10 in a counterclockwise direction and its surfaceis engaged by the deflected end 12 of trailing blade 13' clamped betweentwo sections 16 and 17 of a hopper. The fluid coating composition, Sc,which is to form the subcoat is fed onto the web surface ahead of thetrailing blade 13' from a plurality of tubes 60, only one of which isshown, spaced across the web. The subcoat material is fed into thesetubes by a pump, not shown, and the streams of coating reaching thebackside of the deflected end of the trailing blade merge into a puddle21' from which the material flows under the blade and is trowelled intoa layer L on the web surface thereby.

As in the H6. 2 embodiment the fluid coating composition, Sm, which isto form the middle coat is pumped into a cavity 22 in the hopper section16 by a constant discharge pump and issues therefrom through a dischargeslot 24' in the form of a layer L, which is directed onto the uppermostsurface of the trailing blade. The fluid coating composition, St, whichis to form the top coat is fed into a cavity 42 in another hoppersection 43 by a pump of the constant discharge type. This coatingcomposition issues in the form of a layer L from a discharge slot 45defined by a surface 46 on the hopper section 43 spaced from the top ofhopper section 23'. This layer flows down the inclined surface 47 ofhopper section 23 and onto the top of the layer L, of the middle coatissuing from cavity 22'. The two layers L and L then flow down theuppermost surface of the trailing blade in distinct layer relation andflow of? the end of the blade onto the layer L of subcoat while stillmaintaining a distinct layer relationship with each other and thesubcoat. The final thickness of the individual layers L, and ls aredetermined, not by the width of the discharge slots through which theyissue from and supply cavities 22' and 42, respectively, but by the rateat which they are pumped into their respective cavities. The coveragethickness of the subcoat will depend upon the smoothness of the websurface. the physical properties of the coating composition used and thepressure exerted by the trailing blade in trowelling the coating.

The blade edge geometry has a major influence on the ability of thetrailing blade slide coater process to effectively coat a continuousweb. FIGS. 6, 7 and 8 show these types of edge geometries used in trialsof this method and apparatus of coating. The blade edge shown in FIG. 8was found to be best suited for use with the trailing blade slide coaterprocess.

The edge shown in FIG. 6 is well suited for applying overcoat layerssince the sharp edge makes it easy for the overcoat composition(s) toslide onto the web surface. A disadvantage of this type of blade edge isits high sensitivity to any paper fibers (if the web is a raw paperstock) or foreign particles that may be present on the web surface. Thefibers and foreign particles are retained at the blade edge and causestreaks in the coating. The sharp edge blade tends to wear quicklycausing additional problems.

The blade edge as shown in FIG. 7 is well suited to applying a bladecoat to a continuous web. The rounded edge will apply a coating that hasa good finish, and it is not largely influenced by dirt and paperfibers. A disadvantage with this blade is that it becomes extremelydifficult to slide and overcoat ofi the blade and onto a continuous web.

The blade as shown in FIG. 8 is a compromise between those shown inFIGS. 6 and 7. It is designed to utilize the advantages of both theknife edge blade and the rounded edge blade. in a series of coatingexperiments performed with the trailing blade slide coater, it was foundthat the coating and trailing blade angles (see FIG. I) had no majorinfluence on the coating technique. Both angles were varied throughlarge ranges.

The trailing blade slide coating technique has been evaluated on acommercial coating machine using aqueous solutions of gelatin, methanolsolutions of polyvinyl acetate (hereinafter referred to as PVA), aqueoussolutions of carboxymethyl cellulose (hereinafter referred to as CMC)and aqueous dispersions of baryta and aqueous dispersions of claycaseinas the blade coated materials. Aqueous solution of gelatin, PVA and CCwere also used for the slide coat. Satisfactory coverages ranged from0.06 to 1.40 lb./l ft. for the slide coat and 0.08 to 0.50 lb./l00 ft.for the blade coat. Coating speeds ranged from 200 to 1,030 ft./min.,this high speed being the upper limit which the machine used was capableof. In some cases dyes were mixed into the coating fluids so thatvarious layers of a coating sample could be differentiated. For themajority of these trials a raw paper stock was used as the continuousweb. Paper coated with polyethylene or baryta were also tried in orderto determine 5 the effect of precoated paper webs on this coatingtechnique.

The following tables list the experimental details of different trailswhich were made to evaluate possible limitations of the present coatingtechnique. Table 1 lists trials in which only the blade coat, orsubcoat, was applied to the continuous web. Table 2 lists trials inwhich the blade coat and the overcoat (slide coat) were appliedsimultaneously. Triple coats applied in a like manner are listed intable 3. Table 4 gives the viscosities for the fluid compositions used.

5 TABLE 1 Samples of Subcoat Trials Only Trinl Material Solids WetCoverage Machine Speed 1% lb./l00 n. flJmin.

l Gelatin 5 0.15 050 2 Gelatin I5 032 260 3 Gelatin" ls 0.35 200 4Gelatin" I5 012 050 s Gelatin is 0.50 050 s Gelatin I5 0.35 300 7Gelutin 20 0.14 300 8 Gelatin 25 0.40 020 9 Gelatin 0.13 200 30 I0 CMC 3016 300 ll CMC 2; 0|: 910

l2 PVA 20 0.l5 350 t3 PVA [2 0.50 am i4 buryta 64 0.28 310 I5 barytu s40.ll 3l0 I6 clay-casein 45 0.") 3H) I7 clay-casein S7 014 300'Polycouted paper stock 'Baryta-coated paper stock All other trials rawpaper stock. 40

TABLE 2.SAMPLES WITH SUB-COAT AND OVERCOAT Sub-coat Overcoat Wet Wet;Machine Solids, coverage, Solids, coverage. speed, Trial Materialpercent lb./100 it. Material percent lb./100 it. it./min.

15 5 1. 00 200 15 5 0. 29 800 15 5 0.11 500 15 5 0.11 500 I5 5 0.20 85015 5 0.30 200 20 5 0. 300 20 5 0. 20 300 20 5 0. 17 920 25 5 0. 06 103025 5 0.18 620 3 5 0. 06 300 3 5 0.12 990 3 5 0.09 950 3 5 0.15 950 3 50. 22 950 3 20 0. 54 940 3 20 0.44 670 3 12 0. 30 370 3 18 0.09 300 3 180. 16 300 3 18 0. 23 950 3 18 0.10 950 8 5 0. 20 310 8 5 0. 06 980 8 50. I0 080 8 5 0. 16 980 B 3 0. 00 970 R 3 0.15 J 8 :l 0.35 070 ii 3 0.Ill 1400 H :l 0, 34 mill )4 :l ll. I7 050 ll :4 0.20 510 I2 3 0. all 30020 :l 0. till 300 20 llt 0. 02% 050 04 r) 0.10 310 ($4 5 0.11 050 1Baryta-coated paper stock. 2 Polycoated paper stock.

Norm-All other trials raw paper stock.

TABLE 3.-SAMPLES WITH SUB-COAT, MIDDLECOAT AND OVERCOAT supine:

Middlecoat Overcoat Machine Solids, Coverage, Solids, Coverage, Solids,Coverage, speed, it./ Trial Material percent lb./1OO L Material percentlb./l it. Material percent lbJlOO It. min.

20 0.15 Gelatin 5 0.70 Gelatin 5 0.70 300 3 0 0-.. 5 0.54 do 5 0.54 3003 Clay-casein 57 0 14 -.do .r 18 0.54 do 18 0.54 300 NOTE.All trials onraw paper stock.

TABLE 4 Viscosities of Coating Compositions Viscosity testingtemperature: l00 F.

l. Subcoat (blade coat) Fluid Compositions. (See FIG. I, Table l.)

A. Aqueous Gelatin Solutions in contrast to elastic solids, most liquidsare considered to possess no elasticity or rigidity when sheared andshould respond immediately to any attempt made to deform them. Thereare, however, liquids that generate measurable normal stresses whensheared and show both viscous and elastic effects. These liquids arecalled viscoelastic solutions or dispersions.

Windle and Beazley, The Role of Viscoelasticity in Blade Coating, Tappi,Aug. l968, Vol. 5, No. 8, shows that viscoelasticity has a largeinfluence on blade coating processes. in most cases, the process becomesunstable when viscoelastic effects become large. In this set ofexperiments, the aqueous gelatin solutions ranging from 5 percent to 20percent solids worked very well for the subcoat application. However,when the solids content was increased above 20 percent, the viscoelasticproperty of the solution became more prominent, with the result that theblade tended to lift off the paper and the coating system becomeunstable. Therefore, it is felt that the high solids gelatin solutionsdo not lend themselves readily to blade coat applications.

B. Methanol Solution of Polyvinyl Acetate (PVA) The l2 percent solidsPVA solution worked well as an undercoat material. However, the 20percent solids PVA solution exhibited a pronounced viscoelastic effectand caused some instability of the blade coat. A methanol solution ofpolyvinyl acetate does not lend itself to use with an aqueous overcoat,because upon drying, the methanol escapes from the PVA and causes spotson the overcoats.

C. High-Solids Clay-Casein and High-Solids Baryta Both of these mixturesworked extremely well as subcoat fluids. The clay and baryta dispersionsare highly shear sensitive and thin quite readily in a blade coatingprocess.

D. CMC

The CMC (3 percent to 8 percent solids) worked well as a subcoatsolution. This material exhibits the same shear thinning qualities asthe high-solids clay and baryta materials. ll. Overcoat (blade coat)Fluid Compositions A. Gelatin The solids content of the gelatinsolutions used for the overcoat ranged from 5 percent to 18 percent. Allmixtures that were used worked very well. In some instances, a wettingagent was added to these mixtures to improve their wetting qualities.Although other wetting agents probably could have been usedsatisfactorily, the one used was that sold under the trademark TritonX-200 and having the chemical name P- l, l 33-tetramethylbutyl)phenoxyethoxyethyl sodium sulfonate.

B. PVA

The PVA mixtures l2 percent and 20 percent) solids were found to be goodovercoat fluids. There was some slight difficulty in coating this fluidcomposition because of its quickdrying characteristics.

C. CMC

The 3 percent solids CMC was a very poor overcoat solution. The materialwould not distribute evenly as it was transferred from the blade to thecontinuous web and acted much like an elastic material. This problem wasfound to be characteristic of the CMC solution and not a limitation ofthe trailing blade slide coater process. The 8 percent solids CMC wasnot tried as an overcoat fluid due to the problems associated with thelower solids mixture.

Microsections of various coated specimens made in the described trialswere made. It was found that subcoats of low solid mixtures were hard todetect on the sections examined. The blade coater, when operating in thecorrect manner, will apply a very small amount of solution to precoatedpaper webs. For this reason, low-solids mixtures such as 3 percent CMCand 5 percent gelatin are extremely hard to detect. High-solids mixturessuch as the clay and baryta were more easily seen. It is evident fromthe sections examined that the high-solids mixtures fill in the lowareas of the continuous web and act as good surface conditioners.

Microsections of the three-layer coatings were examined to determine ifthe coating techniqge was capable of applying two overcoat layers thatwould remain separated after drying. The apparatus used in applying thethree layer coatings is shown in FIG. 5 and the data of the test isdetailed in table 3. The sections examined showed separation of thethree layers from one another.

As tables l, 2, and 3 show, the trailing blade slide coating process wastested at speeds ranging from 200 to 1,030 feet per minute. The upperlimit is not the coating process limit but the maximum speed at whichthe machine on which the tests were made could be safely run. Based onexperimental tests, it can be stated that the trailing blade slidecoating process is capable of speeds in excess of 1,000 feet per minute.

For the tests shown in tables 1, 2 and 3, raw paper, precoated barytapaper and polyethylene-coated paper were used as the continuous webmaterial, and all were coated successfully. A major drawback of thetrailing blade slide coater is its sensitivity to dust and paper fiberson the paper web which causes the coatings to be streaked. This problemwas more evident in the raw paper coatings because the paper and dustparticles were more numerous. As the blade contacts the paper web,foreign particles and paper fibers are contained by the blade edges.After a long period of coating, these particles build up and causestreaked finishes. Cleaning the raw paper web before coacting didalleviate a large part of the problem and helped improve the coatingfinish. Using a precoated stock also helped to diminish streaks andimprove the finish. It is apparent from these experiments that if theweb being coated were a plastic film base, rather than paper, thisstreaking problem would have been less troublesome, particularly if thefilm surface was cleaned just ahead of the coating point. For a givenblade and hopper setting, the precoated paper stock allowed more subcoatfluid to be applied to the web than a raw paper stock because thesubcoat cannot be forced into the stock by the trowelling action of theblade. Consequently, for a given setting of the blade, more of thesubcoat fluid is left on top of the web, because this situation causesmore hydrostatic pressure to be applied to the blade tip by the subcoatfluid.

In the trials carried out, the coating and trailing blade angles had nomajor influence on the coating technique. Both angles were variedthrough large ranges.

Although examples of certain synthetic polymeric solvent solutions,aqueous pigment dispersion. and aqueous colloidal solutions which can becoated by the present trailing blade slide coater technique as single ordistinct multiple layers have been disclosed it will be obvious thatthere are many other fluid compositions which can be satisfactorilycoated by this technique. For example, an aqueous solution of polyvinylalcohol could be coated just as satisfactorily as the mentioned solventsolution of polyvinyl acetate. This material could be satisfactorilyused as a subcoat under an overcoat of an aqueous solution of gelatin,while the methanol solution of polyvinyl acetate could not because ofthe methanol causing spots in the overcoat as it escapes during drying.Also, since aqueous gelatin solutions have been shown to coatsatisfactorily by the use of this technique it would be reasonable toassume that (photographic) gelatino silver halide emulsions and othercoatings used in the photographic art could be readily multiple coatedby this technique, see U.S. Pat. No. 2,761,791, Russell, Sept. 4, 1956.It will also be apparent from the disclosure of U.S. Pat. No. 2,761,791that more than three layers of the same or different fluid coatingcompositions could be simultaneously applied to the surface of a web bythis trailing blade slide coater technique by merely combining moreslide hoppers with the blade coater.

Although it is conceivable that the hoppers shown in FIGS. 2-5 could bemade from a single block of material, in order to facilitate thefabrication of the same and to make it possible to clean it out when itis desired to change from one coating composition to another, it is morepractical to make the hopper up from a number of separate sections, asshown, which can be readily assembled and disassembled. lt should alsobe mentioned that if the coating compositions are of such a nature thatthey to be heated or cooled while in the hopper in order to keep them ina suitable condition for flow, then the hopper sections may be providedwith bores through which a heating or cooling liquid may be circulated,as is well known in the art.

This invention has been described with reference to particularembodiments thereof but it will be understood that variations andmodifications may be effected within the spirit and scope of theinvention.

lclaim:

l. The method of applying a thin layer of uniform thickness of a fluidcoating composition onto the surface of a web comprising the steps of:

a. continuously advancing the web while supporting it in a smoothcondition;

b. applying a fluid subcoating composition to the surface of said web;

c. trowelling said subcoating composition on the web surface by movingthe web while supported past the flexible end of a stationary trailingblade pressing against the web surface to fill any low areas in the websurface and provide a smooth, wet surface on said web;

d. continuously metering a given quantity of said fluid coatingcomposition in the form of a layer onto the uppermost surface of saidtrailing blade at a point spaced above the flexible end thereof, saidlayer of fluid coating composition, under the influence of gravity,flowing down said trailing blade to form a uniformly thin layer whichultimately flows ofi the flexible end of said blade and onto the surfaceof said subcoat; and

e. simultaneously curing said subcoat and the thin layer of fluidcoating composition.

2. The method of applying three fluid coating compositions to thesurface of a web in superposed distinct layer relationship comprisingthe steps of:

a. continuously advancing the web while supporting it in a smoothcondition;

b. applying a first fluid coating composition to the surface of saidweb;

c. trowelling said first coating composition on the surface of the webby moving the web while supported past the flexible end of a stationarytrailing blade pressing against the web surface to apply a thin layer ofsaid first coating composition on the web surface;

d. continuously metering a given quantity of the second coatingcomposition in the form of a layer onto the uppermost surface of saidtrailing blade at a point spaced above the flexible end thereof;

e. continuously metering a given quantity of the third fluid coatingcomposition through a slot and onto a slide surface inclined downwardlyand intersecting the layer of said second coating composition at anacute angle and at a point adjacent the point where said layer of secondcoating composition meets said blade surface whereby said layer of thirdcoating composition flows onto the top of said second layer of coatingcom osition and the two layers gravitate own said blade an off theflexible end thereof onto the top of the layer of first coatingcomposition trowelled on the wet surface; and simultaneously curing saidthree layers of coating composition deposited on said web surface.

2. The method of applying three fluid coating compositions to thesurface of a web in superposed distinct layer relationship comprisingthe steps of: a. continuously advancing the web while supporting it in asmooth condition; b. applying a first fluid coating composition to thesurface of said web; c. trowelling said first coating composition on thesurface of the web by moving the web while supported past the flexibleend of a stationary trailing blade pressing against the web surface toapply a thin layer of said first coating composition on the web surface;d. continuously metering a given quantity of the second coatingcomposition in the form of a layer onto the uppermost surface of saidtrailing blade at a point spaced above the flexible end thereof; e.continuously metering a given quantity of the third fluid coatingcomposition through a slot and onto a slide surface inclined downwardlyand intersecting the layer of said second coating composition at anacute angle and at a point adjacent the point where said layer of secondcoating composition meets said blade surface whereby said layer of thirdcoating composition flows onto the top of said second layer of coatingcomposition and the two layers gravitate down said blade and off theflexible end thereof onto the top of the layer of first coatingcomposition trowelled on the wet surface; and f. simultaneously curingsaid three layers of coating composition deposited on said web surface.